My loose (and, obviously, I am not a lawyer) understanding is that under US law the kernel could be considered either a joint work or a collective work. In a joint work I believe that any copyright holder could enforce the copyright of the entire work - but the flipside of that is that any copyright holder could also grant a non-exclusive license to anyone else, absent contractual obligations. In that case anyone holding copyright on a portion of the kernel could grant permission for the entire kernel to be used under terms other than the GPL.
It seems more likely that the kernel would be considered a derivative work, ie one in which each author only holds copyright over their specific contributions. I don't know that that's ever been tested in court, but nor am I busy establishing a business case for licensing the kernel to Microsoft in return for a couple of yachts.